All of the world’s major deserts are predicted to expand, with the Sahara reaching right into central Europe. Glacial retreat will dry Europe’s rivers from the Danube to the Rhine, with similar effects in mountainous regions including the Peruvian Andes, and the Himalayan and Karakoram ranges, which as result will no longer supply water to Afghanistan, Pakistan, China, Bhutan, India and Vietnam. Along with the exhaustion of aquifers, all this will lead to two latitudinal dry belts where human habitation will be impossible, say Syukuro Manabe of Tokyo University, Japan, and his colleagues. One will stretch across Central America, southern Europe and north Africa, south Asia and Japan; while the other will cover Madagascar, southern Africa, the Pacific Islands, and most of Australia and Chile.

The first problem would be that many of the places where people live and grow food would no longer be suitable for either. Rising sea levels – from thermal expansion of the oceans, melting glaciers and storm surges – would drown today’s coastal regions in up to 2 meters of water initially, and possibly much more if the Greenland ice sheet and parts of Antarctica were to melt. “It’s hard to see west Antarctica’s ice sheets surviving the century, meaning a sea-level rise of at least 1 or 2 meters,” says climatologist James Hansen, who heads NASA’s Goddard Institute for Space Studies in New York. “CO2 concentrations of 550 parts per million [compared with about 385 ppm now] would be disastrous,” he adds, “certainly leading to an ice-free planet, with sea level about 80 meters higher… and the trip getting there would be horrendous.”

Half of the world’s surface lies in the tropics, between 30° and -30° latitude, and these areas are particularly vulnerable to climate change. India, Bangladesh and Pakistan, for example, will feel the force of a shorter but fiercer Asian monsoon, which will probably cause even more devastating floods than the area suffers now. Yet because the land will be hotter, this water will evaporate faster, leaving drought across Asia. Bangladesh stands to lose a third of its land area – including its main bread basket. The African monsoon, although less well understood, is expected to become more intense, possibly leading to a greening of the semi-arid Sahel region, which stretches across the continent south of the Sahara desert. Other models, however, predict a worsening of drought all over Africa. A lack of fresh water will be felt elsewhere in the world, too, with warmer temperatures reducing soil moisture across China, the south-west US, Central America, most of South America and Australia.

So if only a fraction of the planet will be habitable, how will our vast population survive? Some, like Lovelock, are less than optimistic. “Humans are in a pretty difficult position and I don’t think they are clever enough to handle what’s ahead. I think they’ll survive as a species all right, but the cull during this century is going to be huge,” he says. “The number remaining at the end of the century will probably be a billion or less.”

When, and if, we get 4 degrees hotter depends not only on how much greenhouse gas we pump into the atmosphere and how quickly, but how sensitive the world’s climate is to these gases. It also depends whether “tipping points” are reached, in which climate feedback mechanisms rapidly speed warming. According to models, we could cook the planet by 4 °C by 2100. Some scientists fear that we may get there as soon as 2050. If this happens, the ramifications for life on Earth are so terrifying that many scientists contacted for this article preferred not to contemplate them, saying only that we should concentrate on reducing emissions to a level where such a rise is known only in nightmares.

This will probably be a mostly vegetarian world: the warming, acidic seas will be largely devoid of fish, thanks to a crash in plankton that use calcium carbonate to build shells. Molluscs, also unable to grow their carbonate shells, will become extinct. Poultry may be viable on the edges of farmland but there will simply be no room to graze cattle. Livestock may be restricted to hardy animals such as goats, which can survive on desert scrub. One consequence of the lack of cattle will be a need for alternative fertilisers – processed human waste is a possibility. Synthetic meats and other foods could meet some of the demand. Cultivation of algal mats, and crops grown on floating platforms and in marshland could also contribute.

While the exact changes would depend on how quickly the temperature rose and how much polar ice melted, we can expect similar scenarios to unfold this time around.

The only places we will be guaranteed enough water will be in the high latitudes. “Everything in that region will be growing like mad. That’s where all the life will be,” says former NASA scientist James Lovelock, who developed the “Gaia” theory, which describes the Earth as a self-regulating entity. “The rest of the world will be largely desert with a few oases.”